It is often important in power plants and process plants to verify the thrust and torque developed in the valve stems of motor operated valves. It may be to assure that the valve develops sufficient stem thrust to be able to close off the flow in an emergency situation, or it may be to assure that the stem lubrication is still good. Stem lubrication will be good if the torque to thrust ratio, or stem factor, has not increased unduly over time.
U.S. Pat. Nos. 4,805,451 and 4,911,004 describe a method and apparatus for measuring valve stem thrust. The apparatus employs a special extensometer affixed to the valve yoke. The extensometer is calibrated by temporarily affixing a special diametral strain sensing device to the valve stem just as the valve is seated. An important advantage of the method is that it can be universally applied to valves having all types of exposed stems, both threaded and unthreaded. Nevertheless, the method only measures stem thrust, not stem torque, and it measures stem thrust with accuracies not much better than .+-.10%.
For a subset of the above total population of motor operated valves, it is possible to apply strain gages to the stem because the stems have an unthreaded (smooth) portion which remains exposed throughout the total valve stroke. Strain gages allow both torque and thrust to be measured. U.S. Pat. No. 5,347,871 describes a single strain gage coupon which is bonded to a valve stem. The coupon contains strain gage elements that form two separate and independent bridge circuits, one for thrust and one for torque. The coupon must be substantially longer than half the circumference of the valve stem so that a spring clamp temporarily attached to protuberances at its ends will exert both circumferential and radial forces for bonding the coupon to the valve stem. Also, the coupon length must be specific to each particular valve stem diameter so that certain of the elements will be spaced 180 degrees apart on the stem so as to cancel out any stem bending effects.
Ideally, the strain gage bridges should be calibrated after being bonded to the valve stem. That is, their outputs should be checked against a known thrust input and a known torque input. In practice, this is difficult to do for the thrust, and even more difficult to do for the torque. The end result is that often these calibrations are not performed. Instead, one just computes the sensitivities of the thrust bridge and the torque bridge using the known gage factors of the strain gage elements, the known diameter of the stem, and the known properties of the stem material. By computing these sensitivities rather than calibrating to obtain them, one effectively assumes that the bond beneath each strain gage element is perfect. If the bond beneath each strain gage element is indeed perfect, then accuracies in the order of .+-.5% should be achievable. The problem, though, is that sometimes the bonds are not perfect, and instead of achieving about .+-.5% accuracy, the computed sensitivities may be inaccurate by .+-.25%, .+-.50%, or even more. Worse yet, with independent thrust and torque bridges, when the bond beneath a strain gage element is not perfect, the user has no way of knowing that.
Accordingly, there is still a need for technique to verify the bond integrity beneath each strain gage so that true .+-.5% accuracies can be achieved without having to perform a very difficult calibration procedure where known thrusts and known torques must be applied. The technique should be readily applicable to the valve stem of motor operated valves. The present invention fills such needs by providing methods of employing strain gages whose signal outputs are "directly employed" to verify the bond integrity, and if the bond integrity is acceptable, the signal outputs are employed to make thrust and torque measurements and determine stem bending moments.